1. Field
Example embodiments in general relate to a battery pack configured for powering tools of a cordless power tool system having an arrangement of internal components within a housing thereof, to an internal component arrangement for a battery pack and to a methodology for arranging the internal components within the battery pack.
2. Description of Related Art
Cordless products or devices which use rechargeable batteries are prevalent throughout the workplace and home. Rechargeable batteries may be used in numerous devices, from computer products and/or housewares to power tools. Nickel-cadmium, nickel-metal-hydride battery and/or lithium-ion cells may be used in these devices. Since the devices use a plurality of battery cells, the battery cells may be ordinarily packaged as battery packs. These battery packs may be coupled with the cordless devices so as to secure the pack to the device. The battery pack may be removed from the cordless device and charged in a battery charger or charged in the cordless device itself, for example.
As battery technologies become more advanced, it is increasingly desirable to have intelligent battery packs for these cordless devices, such as cordless power tools, which are capable of self-monitoring. This self-monitoring feature necessitates electronics and sensors to be disposed within the battery pack. Current battery pack designs, such as those designed for cordless power tools and associated chargers of a cordless power tool system, do not typically provide an adequate support structure/housing to mechanically retain all of these components and the battery cells.
For example, in a multiple cell battery pack, there is a need to electrically connect cells to one another. This is typically accomplished by welding electrically conductive cell straps between cells. These weld joints, which occur between the cell cans and the electrically conductive straps, are critical to the operation and performance of the battery pack. Because of the importance of these welds, the manufacturing process should be tightly controlled.
A bad weld can result in an open-circuited battery pack, which can be detected by an end of line tester and either scrapped or re-worked, increasing fabrication cost. A marginal weld could fail in the field, either causing an open circuited pack or a high-impedance pack. The user would either notice a non-functional battery pack or a pack with decreased performance.
The manufacturing process of locating and restraining the multitude of straps needed for fabricating an individual battery pack can be difficult, requiring additional fixtures and cost. Conventionally, the manufacturer creates a jig to hold the cells in position and another fixture (or mask) to hold the straps in position relative to the cells during welding. Once the welds are complete, the mask and the jig are removed and the resulting “core-pack” (e.g., cells held together by their welded cell straps) is inserted into its housing.
This manufacturing technique may cause residual stresses on the weld joints. The cells are constrained in one position while in the manufacturing jig. The welds are applied with the cells constrained in one position while in the manufacturing jig. The jig is then removed and the cells are free to move. Then the cells are inserted into a pack housing, forcing the cells into a different position or orientation. In other words, the positioning of the cells in the pack housing may not be exactly the same positioning with which the cells were welded, which places stresses on the weld joints at strap-to-can interfaces. During operating of the pack in a system, such as when attached to a cordless power tool, and upon operation-induced vibration and/or accidental dropping of the tool, these weld joints are more likely to fail because of residual stresses introduced during the assembly of the internal components within the pack housing.
Further, a smart battery pack typically may require a plurality of signal-level conductors throughout the battery pack. These conductors carry information about the status of the pack to a control unit in the pack which may be a microprocessor or microcontroller. Because this information is gathered from different locations within the battery pack, the wire-up of these conductors can pose a challenge to manufacturers.
As an example, in a smart battery pack capable of self-monitoring, each cell's voltage is individually monitored by a controller in the pack, such as a microprocessor, microcontroller, etc. This requires that each cell be wired up to the controller. Because of the low current nature of these signals, thin gage wire could be used as the signal-level conductors, as it takes up less space within the pack. Using thin gage wire, however, presents challenges in a power tool environment. A power tool battery pack can experience high vibration in operation, such as during operation of a cordless reciprocating saw as well as severe mechanical shock, such as a user dropping a tool off of a multistory building. These scenarios are likely to lead to failure of thin gage wire that is soldered or ultrasonically welded (or otherwise rigidly attached) to the cells.